Multicomponent viscoelastic surfactant fluid and method of using as a fracturing fluid

ABSTRACT

There is a viscoelastic fluid. The fluid has one or more cationic surfactants selected from the group consisting of certain quaternary salts, certain amines, and combinations thereof; one or more anionic polymers/anionic surfactants; one or more of certain zwitterionic/amphoteric surfactants; and water. There is also a method of fracturing a subterranean formation. The viscoelastic fluid is pumped through a wellbore and into a subterranean formation at a pressure sufficient to fracture the formation. There is also a method for gravel packing a subterranean formation.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation-in-part application of U.S.Ser. No. 11/141,853, filed Jun. 1, 2005, now abandoned, which claimspriority based on U.S. Provisional Application 60/576,124, filed Jun. 2,2004, now expired.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a viscoelastic fluid. The presentinvention further relates to a method of fracturing a subterraneanformation with a viscoelastic fluid.

2. Description of the Related Art

Viscoelastic surfactant (VES) fluids have continued to grow in use inoilfield applications because of their advantages over conventionalpolymer systems. Such advantages include higher permeability in the oilbearing zone, lower formation or subterranean damage, higher viscosifierrecovery after fracturing, elimination of need for enzymes or oxidizersto break down viscosity, and easier hydration and faster build-up tooptimum viscosity.

Growth in the use of VES fluids has been inhibited by the high cost ofsurfactants required to formulate such fluids. Another problem with useof VES fluids is their low tolerance of organic/inorganic salts and claystabilizers, such as potassium chloride and tetramethyl ammoniumchloride (TMAC), in subterranean formations. Another problem with use ofVES fluids is the high temperatures encountered in deep well oilfieldapplications, i.e. up to 250° C. High temperatures can break down theviscosity of VES fluids and render them ineffective in fracturingoperations when viscoelastic surfactants are present at lowconcentrations or require use of high viscoelastic surfactantconcentrations to avoid such viscosity breakdown. Use of viscoelasticsurfactants at low concentrations also can result in unacceptably longshear recovery time after high shear operation.

In the prior art, attempts have been made to remedy breakdown inviscosity performance by adding polymers and/or cosurfactants, such aslow molecular weight anionic polymers. However, shear recovery can beunacceptably long and/or organic/inorganic salt tolerance may beinadequate.

Accordingly, it would be desirable to have a VES fluid that could beformulated on a cost-effective basis, i.e., with relatively low levelsof viscoelastic surfactant. It would further be desirable to have a VESfluid that exhibits high tolerance with respect to organic/inorganicsalts and clay stabilizers. It would still further be desirable to havea VES fluid with relatively low levels of viscoelastic surfactant thatmaintains a high level of viscosity performance at high temperatures andshear recovery comparable to fluids with a relatively high concentrationof viscoelastic surfactants.

SUMMARY OF THE INVENTION

It is an object of the present invention to have a viscoelastic fluid.

It is another object of the present invention to have a viscoelasticfluid useful in oilfield applications.

It is still another object of the present invention to have aviscoelastic surfactant fluid that can be formulated with a relativelylow level of surfactant for cost-effective performance.

It is a further object of the present invention to have a viscoelasticfluid with high tolerance to organic/inorganic salts, such as KCl andTMAC and Ca++ and Mg++ ions.

It is yet a further object of the present invention to have aviscoelastic fluid that maintains a high level of viscosity performanceat high temperatures.

It is a further objective of the present invention to have aviscoelastic surfactant fluid that exhibits good shear recovery afterhigh shear operation.

According to this and other objects and advantages of the presentinvention, there is provided a viscoelastic fluid. The fluid has one ormore selected cationic surfactants, one or more selected anionicpolymers and/or anionic surfactants, one or more selected zwitterionicand/or amphoteric surfactants, and water.

The cationic surfactant is selected from i) certain quaternary salts andii) certain amines, iii) certain amine oxides, iv) and combinationsthereof.

The quaternary salts have the structural formula:

wherein R₁ is a hydrophobic moiety of alkyl, alkylarylalkyl,alkoxyalkyl, alkylaminoalkyl or alkylamidoalkyl, and wherein R₁ has fromabout 12 to about 25 carbon atoms and may be branched orstraight-chained and saturated or unsaturated.

R₂, R₃, and R₅ are, independently, an aliphatic moiety having from 1 toabout 30 atoms or an aromatic moiety having from 7 to about 15 atoms.The aliphatic moiety can be branched or straight-chained and saturatedor unsaturated.

X is suitable counter-anion, such as Cl⁻, Br⁻, and Ch₃CH₃SO4⁻.

The amines have the following structural formula:

-   -   wherein R₁, R₂ and R₃ are as defined above.    -   The amine oxides have the following structural formula:

-   -   wherein R₁, R₂ and R₃ are as defined above.

The anionic polymer has about 8 to about 100 monomeric units and atleast one negatively charged moiety. Sulfonated polymers are preferred.Anionic surfactants will have alkyl chains of about 6 to about 18 carbonatoms with at least one negatively charged moiety.

The zwitterionic surfactant has the following structural formula:

wherein R₁, R₂, and R₃ are as described above. R₄ is a hydrocarbylradical with a chain length of 1 to 4 carbon atoms.

The amphoteric surfactant has the following structural formula:

wherein R₁, R₂, and R₄ are as described above.

Further according to the present invention, there is provided a methodof fracturing a subterranean formation, comprising the step of pumpingthe viscoelastic fluid through a wellbore and into a subterraneanformation at a pressure sufficient to fracture the formation.

Still further according to the present invention, there is provided amethod of gravel packing a subterranean formation, comprising the stepof pumping the viscoelastic fluid and gravel into a wellbore.

DESCRIPTION OF THE FIGURES

FIG. 1 is a plot diagram of the viscosity profile as a function oftemperature for fluids of the present invention versus a comparisonfluid.

FIG. 2 is a plot diagram of the viscosity profile as a function oftemperature for a fluid of the present invention versus comparisonfluids.

DETAILED DESCRIPTION OF THE INVENTION

The property of viscoelasticity in general is well known, and referenceis made to S. Gravsholt, Journal of Coll. And Interface Sci., 57(3), 575(1976); Hoffmann et al., “Influence of Ionic Surfactants on theViscoelastic Properties of Zwitterionic Surfactant Solutions”, Langmuir,8, 2140-2146 (1992); and Hoffmann et al., “The Rheological Behaviour ofDifferent Viscoelastic Surfactant Solutions,” Tenside Surf. Det., 31,389-400, 1994. Several test methods have been specified in thesereferences to determine whether a liquid possesses viscoelasticproperties. One test that has been found useful in determining theviscoelasticity of an aqueous solution is swirling the solution andvisually observing whether the bubbles created by the swirling recoilafter the swirling is stopped. Any recoil of the bubbles indicatesviscoelasticity. Another useful test is to measure the storage modulus(G′) and the loss modulus (G″) at a given temperature. If G′>G″ is atsome point or over some range of points below about 10 rad/sec,typically between about 0.001 to about 10 rad/sec, more typicallybetween about 0.1 and about 10 rad/sec, at a given temperature and ifG′>10⁻² Pascals, preferably 10⁻¹ Pascals, the fluid is typicallyconsidered viscoelastic at that temperature. Rheological measurements,such as G′ and G″, are discussed more fully in “RheologicalMeasurements”, Encyclopedia of Chemical Technology, vol. 21, pp.347-372, (John Wiley & Sons, Inc., N.Y., N.Y., 1997, 4th ed.).

Viscoelasticity is caused by a different type of micelle formation thanthe usual spherical micelles formed by most surfactants. Viscoelasticsurfactant fluids form worm-like, rod-like or cylindrical micelles insolution. The formation of long, cylindrical micelles creates usefulrheological properties. The viscoelastic surfactant solution exhibitsshear thinning behavior, and remains stable despite repeated high shearapplications.

The viscoelastic surfactants useful in the fluids of the presentinvention are selected zwitterionic surfactants and/or amphotericsurfactants and cationic surfactants. A zwitterionic surfactant has apermanently positively charged moiety in the molecule regardless of pHand a negatively charged moiety at alkaline pH. A cationic surfactanthas a positively charged moiety regardless of pH. An amphotericsurfactant has both a positively charged moiety and a negatively chargedmoiety over a certain pH range (e.g., typically slightly acidic), only anegatively charged moiety over a certain pH range (e.g., typicallyslightly alkaline) and only a positively charged moiety at a differentpH range (e.g., typically moderately acidic).

The selected viscoelastic surfactants and polymers to form aviscoelastic fluid that exhibits enhanced performance compared toconventional viscoelastic fluids. The viscoelastic fluid of the presentinvention can exhibit high viscosity levels at high temperatures and lowviscoelastic surfactant levels and high tolerance for with respect toCa⁺⁺ ions and clay stabilizers compared to conventional viscoelasticfluids. The viscoelastic fluid of the present invention exhibitsenhanced performance compared to viscoelastic fluids having only thecombination of the selected zwitterionic/amphoteric surfactants andcationic surfactants the combination of the zwitterionic/amphotericsurfactants and the selected anionic polymer/anionic surfactant, or thecombination of the selected cationic surfactants and the selectedanionic polymer/anionic surfactant.

The cationic surfactant is selected from i) certain quaternary salts andii) certain amines, iii) amine oxide, iv) and combinations thereof.

The quaternary salts have the structural formula:

wherein R₁ is a hydrophobic moiety of alkyl, alkylarylalkyl,alkoxyalkyl, alkylaminoalkyl or alkylamidoalkyl. R₁ has from about 18 toabout 30 carbon atoms and may be branched or straight-chained andsaturated or unsaturated. Representative long chain alkyl groups includeoctadecentyl (oleyl), octadecyl (stearyl), docosenoic (erucyl) and thederivatives of tallow, coco, soya and rapeseed oils. The preferred alkyland alkenyl groups are alkyl and alkenyl groups having from about 18 toabout 22 carbon atoms.

R₂, R₃, and R₅ are, independently, an aliphatic group having from 1 toabout 30 carbon atoms or an aromatic group having from 7 to about 15carbon atoms. The aliphatic group preferably has from 1 to about 20carbon atoms, more preferably from 1 to about 10 carbon atoms, and mostpreferably from 1 to about 6 carbon atoms. Representative aliphaticgroups include alkyl, alkenyl, hydroxyalkyl, carboxyalkyl, andhydroxyalkyl-polyoxyalkylene. The aliphatic group can be branched orstraight-chained and saturated or unsaturated. Preferred alkyl chainsare methyl and ethyl. Preferred hydroxyalkyls are hydroxyethyl andhydroxypropyl. Preferred carboxyalkyls are acetate and propionate.Preferred hydroxyalkyl-polyoxyalkylenes are hydroxyethyl-polyoxyethyleneand hydroxypropyl-polyoxypropylene. Examples of aromatic moietiesinclude cyclic groups, aryl groups, and alkylaryl groups. A preferredalkylaryl is benzyl.

X is suitable anion, such as Cl⁻, Br⁻, and (CH₃)₂SO4⁻.

Representative quaternary salts of the above structure includemethylpolyoxyethylene(12-18)octadecanammonium chloride,methylpolyoxyethylene(2-12)cocoalkylammonium chloride, andisotridecyloxypropyl polyoxyethylene (2-12) methyl ammonium chloride.

The amines have the following structural formula:

wherein R₁, R₂, and R₃ are as defined above.

Representative amines of the above structure includepolyoxyethylene(2-15)cocoalkylamines, polyoxyethylene(12-18)tallowalkylamines, and polyoxyethylene(2-15)oleylamines.

The fluid also has an anionic polymer. The polymer has about 8 to about100 monomeric units and at least one negatively charged moiety.Sulfonated polymers are preferred. Representative anionic polymersinclude, but are not limited to, polynapthalene sulfonate, sulfonatedpolystyrenes, and sulfonated styrene/maleic anhydride copolymers. A mostpreferred anionic polymer is polynapthalene sulfonate and has thefollowing structural formula:

wherein n is an integer from about 8 to about 100. Preferredpolynapthalene sulfonates have a weight average molecular weight of fromabout 2,000 to about 20,000.

Another preferred anionic polymer are polyalkylene sulfonates having thefollowing structural formula:

wherein n is an integer from about 8 to about 100. M is an inorganic ororganic cation, such as alkaline metal or ammonium ions, e.g. K⁺, Na⁺,and NH₄ ⁺.

Selected anionic surfactants useful in the viscoelastic surfactant fluidof the present invention include those having alkyl chains of about 6 toabout 18 carbon atoms with at least one negatively charged moiety.

Representative anionic surfactants include those of the followingstructural formulas:

and combinations thereof.

R₆ is selected from a group consisting of alkyl, aryl, alkaryl,alkylarylalkyl, arylalkyl, alkylamidoalkyl, alkylaminoalkyl; wherein thealkyl group has from about 6 to about 18 carbon atoms; wherein the arylgroup represents a phenyl, diphenyl, diphenylether, or naphthalenemoiety; and wherein the total carbon atom content of R₆ is no more thanabout 18 carbon atoms. R₆ is preferably C₁₀ to C₁₈ alkyl oralkylamidoalkyl. R₆ can be represented by octyl, nonyl, decyl, dodecyland the like. Substitutes from natural sources having mixed carbon chainlengths can be used or purified to reduce the number of carbon chainlengths in the alkyl groups. Preferred alkylamidoalkyls are coco/laurylamidopropyl, oleyl/stearyl amidopropyl, octylamidopropyl, anddecylamidopropyl.

M represents hydrogen, an alkali metal such as sodium or potassium, or—[R₇-(EO)_(a)(PO)_(b)(BO)_(c)]_(m)—O—]_(q)—P(O) (OM)₂.

Y represents a counter-ion, which is preferably an alkali metal such assodium or potassium, more preferably sodium; EO represents ethyleneoxyradicals, PO represents propyleneoxy radicals. BO represents butoxyradicals. The letters a, b, and c are, independently, integers from 0 to50, wherein “a” is preferably an integer from 0 to 15 and “b” ispreferably an integer from 0 to 10, and “c” is preferably an integerfrom 0 to 10, wherein EO, PO and BO, radicals can be randomly mixed orin the discrete blocks. “m” is 0 or 1. “R₇” is C₈ to C₁₈ alkylene. R₈ isC₈-C₁₈ alkyl or C₈-C₁₈ alkylamido. “R₉” is C₁-C₄ alkyl or Y(counter-ion). R₁₀ is C₁₀-C₁₄ alkyl. “q” is an integer from 1 to about10.

Selected zwitterionic surfactants useful in the viscoelastic surfactantfluid of the present invention are represented by the followingstructural formula:

wherein R₁ is as described above. R₂ and R₃ are, independently, analiphatic moiety having from 1 to about 30 carbon atoms or an aromaticmoiety having from 7 to about 15 carbon atoms. The aliphatic moietypreferably has from 1 to about 20 carbon atoms, more preferably from 1to about 10 carbon atoms, and most preferably from 1 to about 6 carbonatoms. The aliphatic group can be branched or straight chained andsaturated or unsaturated. Representative aliphatic groups include alkyl,alkenyl, hydroxyalkyl, carboxyalkyl, and hydroxyalkyl-polyoxyalkylene.Preferred alkyl chains are methyl and ethyl. Preferred hydroxyalkyls arehydroxyethyl and hydroxypropyl. Preferred carboxyalkyls are acetate andpropionate. Preferred hydroxyalkyl-polyoxyalkylenes arehydroxyethyl-polyoxyethylene or hydroxypropyl-polyoxypropylene). R₄ is ahydrocarbyl radical (e.g. alkylene) with chain length 1 to 4 carbonatoms. Preferred are methylene or ethylene groups. Examples of aromaticmoieties include cyclic groups, aryl groups, and alkylaryl groups. Apreferred arylalkyl is benzyl.

Specific examples of selected zwitterionic surfactants include thefollowing structures:

wherein R₁ is as described above.

Other representative zwitterionic surfactants include dihydroxyethyltallow glycinate, oleamidopropyl betaine, and erucyl amidopropylbetaine.

Selected amphoteric surfactants useful in the viscoelastic surfactantfluid of the present invention are represented by the followingstructural formula:

wherein R₁, R₂, and R₄ are as described above.

Specific examples of amphoteric surfactants include those of thefollowing structural formulas:

wherein R₁ is as described above. X⁺ is an inorganic cation such as Na⁺,K⁺, NH₄ ⁺ associated with a carboxylate group or hydrogen atom in anacidic medium.

The selected zwitterionic and amphoteric surfactants are functionallyinterchangeable and may be used separately or alone (alternatively) orin combination with each other. Additional teachings to the selectedzwitterionic and amphoteric surfactants are disclosed in U.S. Pat. No.6,258,859 B1, which is incorporated herein in its entirety.

The surfactants are used in an amount, which, in combination with theother ingredients, such as the anionic polymer, is sufficient to form aviscoelastic fluid, which amount will typically be a minor amount byweight of the fluid (e.g., less than about 50% by weight). The totalconcentration of the selected zwitterionic/amphoteric and cationicsurfactants typically ranges from about 0.1 to about 10 wt %, moretypically from about 0.1 to about 5 wt %, and even more typically fromabout 0.25 to about 2 wt % based on the weight of the fluid. The weightpercentage of the cationic surfactant in the total active surfactant(exclusive of solvents) typically ranges from about 1 to about 40 wt %,more typically from about 3 to about 20 wt %, and even more typicallyfrom about 5 to about 20 wt % (based on total required cationicsurfactant and zwitterionic/amphoteric surfactant of the presentinvention). Because of the cost of the surfactants, it is desirable, ifpossible, to minimize surfactant concentration. Most preferably, theconcentration of the selected zwitterionic/amphoteric and cationicsurfactants will be less than 3 wt % based on fluid weight. The selectedanionic polymer is typically used at about 0.01 to about 5 wt %, moretypically about 0.05 to about 3 wt %, and most typically about 0.1 to0.5 wt % based on weight of the fluid. Optimum concentrations for theselected surfactants and anionic polymers can be determinedexperimentally for a particular fluid system.

The viscoelastic fluid is aqueous. Water is preferably present in anamount by weight about 50 percent or more by weight of the fluid. Mostpreferred fluids have about 70 weight percent or more of water by weightof the fluid. The water can be from any source so long as the source hasno contaminants incompatible with the other components of theviscoelastic fluid (e.g., such as to cause undesirable precipitation).Thus, the water need not be potable and may be brackish or contain othermaterials typical of sources of water found in or near oil fields.

The fluid optionally has one or more members from the following group:organic acids, organic acid salts, inorganic salts, and combinationsthereof. This member will typically be present in a minor amount (e.g.,about 20 wt % or less by weight of the fluid).

The organic acid is typically a sulfonic acid or a carboxylic acid andthe anionic counter-ion of the organic acid salts are typicallysulfonates or carboxylates. Representative of such organic moleculesinclude various aromatic sulfonates and carboxylates such as p-toluenesulfonate, naphthalene sulfonate, chlorobenzoic acid, salicylic acid,phthalic acid and the like, where such counter-ions are water-soluble.Most preferred are salicylate, phthalate, p-toluene sulfonate, sodiumxylene sulfonates, hydroxynaphthalene carboxylates, e.g.5-hydroxy-1-naphthoic acid, 6-hydroxy-1-naphthoic acid,7-hydroxy-1-naphthoic acid, 1-hydroxy-2-naphthoic acid, preferably3-hydroxy-2-naphthoic acid, 5-hydroxy-2-naphthoic acid,7-hydroxy-2-naphthoic acid, and 1,3-dihydroxy-2-naphthoic acid and3,4-dichlorobenzoate. The organic acid or salt thereof typically aidsthe development of increased viscosity which is characteristic ofpreferred fluids. Although no bound by any theory, the association ofthe organic acid or salt thereof with the micelle might decrease theaggregation curvature of the micelle, and, thus, promote the formationof a worm-like or rod-like micelle. The organic acid or salt thereofwill typically be present in the viscoelastic fluid at a weightconcentration of from about 0.1 wt % to about 10 wt %, more typicallyfrom about 0.1 wt % to about 7 wt %, and even more typically from about0.1 wt % to about 6 wt % based on fluid weight.

The inorganic salts that are particularly suitable for use in theviscoelastic fluid include water-soluble potassium, sodium, or ammoniumsalts, such as potassium chloride or ammonium chloride. Potassiumchloride is most preferred. Additionally, calcium chloride, calciumbromide and zinc halide salts may also be used. The inorganic salts mayaid in the development of increased viscosity, which is characteristicof preferred fluids. Further, the inorganic salt may assist inmaintaining the stability of a geologic or subterranean formation towhich the fluid is exposed. Formation stability, in particular claystability (by inhibiting hydration of the clay), can be achieved at aconcentration levels of a few percent by weight or less. Thus, densityof fluid is usually not significantly altered by the presence of theinorganic salt. If fluid density is an important consideration, heavierinorganic salts may be used. The inorganic salt will typically bepresent in the viscoelastic fluid at a weight concentration of fromabout 0.1% to about 30%, more typically from about 0.1% to about 10%,and even more typically from about 0.1% to about 8%. Organic salts, e.g.trimethylammonium hydrochloride and tetramethylammonium chloride, mayalso be useful in addition to, or as a replacement for, the inorganicsalts.

As an alternative to the organic salts and inorganic salts, or as apartial substitute therefor, one can use a medium to long chain alcohol(preferably an alkanol), preferably having five to ten carbon atoms, oran alcohol ethoxylate (preferably an alkanol ethoxylate) preferably of a12 to 16 carbon alcohol and having 1 to 6, preferably 1-4, oxyethyleneunits.

The viscoelastic surfactant solution is useful as a fracturing fluid orwater-based hydraulic fluid. The viscoelastic fluid used as a fracturingfluid may optionally contain a gas such as air, nitrogen or carbondioxide to provide an energized fluid or a foam.

When used as a hydraulic fracturing fluid, the viscoelastic fluid maycontain other conventional constituents that perform specific desiredfunctions, e.g., corrosion inhibition, fluid-loss prevention and thelike. A proppant can be suspended in the fracturing fluid. The pH of thefluid will typically range from strongly acidic (e.g., less than a pH ofabout 3) to slightly alkaline (e.g., from a pH just greater than 7.0 toabout 8.5, more typically to about 8.0) or moderately alkaline (e.g., apH of about 8.5 to about 9.5).

The viscoelastic fluid may optionally have one or more additionalthickeners and fluid-loss control additives known in the industry, suchas water-soluble or water-dispersible polymers (guar and guarderivatives, xanthan, polyacrylamide, starch and starch derivatives,cellulosic derivatives, polyacrylates, polyDADMAC [poly(diallyl dimethylammonium chloride] and combinations thereof), and clay (Bentonite andattapulgite).

The viscoelastic fluid may optionally have conventionalsurfactants/cosurfactants other than the viscoelastic surfactantsdescribed above. Such surfactants/cosurfactants can include anionic,cationic, nonionic, zwitterionic, and amphoteric species. The fluid mayalso have any solvent or vehicles known in the art (in addition towater), such as lower monohydric alcohols, polyhydric alcohols, and thelike.

The viscoelastic fluid is useful in conventional hydraulic fracturingmethods. Useful methods are disclosed in U.S. Pat. No. 5,551,516, whichis incorporated by reference. Oil-field applications and methods arealso described in “Oil-field Applications”, Encyclopedia of PolymerScience and Engineering, vol. 10, pp. 328-366 (John Wiley & Sons, Inc.,New York, N.Y., 1987), which is also incorporated herein by reference.

Hydraulic fracturing refers to methods used to stimulate the productionof fluids, such as oil and natural gas, from subterranean formations. Inhydraulic fracturing, a fracturing fluid is injected through a wellboreand against the face of the formation at a pressure and flow rate atleast sufficient to overcome the overburden pressure and to initiateand/or extend a fracture(s) into the formation. The fracturing fluidusually carries a proppant such as 20-40 mesh sand, bauxite, glassbeads, and the like, suspended in the fracturing fluid and transportedinto a fracture. The proppant keeps the formation from closing back downupon itself when pressure is released. The proppant filled fracturesprovide permeable channels through which the formation fluids can flowto the wellbore and thereafter be withdrawn. Viscoelastic fluids havealso been extensively used in gravel pack treatment.

U.S. Ser. No. 11/141,853, filed Jun. 1, 2005 and U.S. ProvisionalApplication 60/576,124, filed Jun. 2, 2004, are hereby incorporatedherein in their entirety.

The following are examples of the present invention. They areillustrative of the invention and are not to be construed as limiting.Unless otherwise indicated, all percentages or parts are by weight.

EXAMPLES

Fluids of the present invention were prepared and tested for viscosityperformance as a function of temperature. Viscosity performance relatedto suitability for use in fracturing applications.

The zwitterionic surfactant employed was erucyl amidopropyl betaine(EAB). The anionic polymer employed was polynapthalene sulfonate (DAXAD19 polymer manufactured by Hampshire Chemical Corp.). The cationicsurfactants employed were methylpolyoxyethylene octadecanammoniumchloride (OAED) and polyoxyethylene cocoalkylamines (CAEO). Allingredients were formulated by mixing.

Two fluids of the following formulations were tested: 27 wt % EAB/3.8 wt% DAXAD 19/3.9 wt % CAEO (balance is solvents); and 27 wt % EAB/3.8 wt %DAXAD 19/3.9 wt % OAED (balance is solvents).

The fluids showed very good viscosity performance at the 2.1 wt %surfactant use level (0.9 wt % active) up to 215° F. with very goodshear recovery (less than 15 seconds). The systems were compatible with2 wt % KCl (potassium chloride), 2 wt % KCl with 500 ppm Ca⁺⁺, 0.1 wt %TMAC (trimethyl ammonium chloride) and 0.1 wt % TMAC with 300 ppm Ca⁺⁺.The three components were formulated together with IPA and water withvery good stability and flowability in the temperature range from 20° F.to 150° F.

Additional fluids were formulated as set forth in FIG. 1. Four fluids ofthe present invention were formulated as follows: I) 1.1 wt % EAB/0.14wt % DAXAD 19/0.15 wt % OAED in 2 wt % KCl (balance is solvents); II)0.76 wt % EAB/0.14 wt % DAXAD 19/0.1 wt % OAED in 2 wt % KCl (rest aresolvents); III) 0.76 wt % EAB/0.14 wt % DAXAD 19/0.1 wt % OAEDin 2 wt %KCl/500 ppm Ca++ (balance is solvents); and IV) 0.76 wt % EAB/0.14 wt %DAXAD 19/0.1 wt % OAED in 0.1 wt % TMAC (balance is solvents). Acomparative fluid was formulated as follows with only 2.3 wt % EAB in 2wt % KCl(balance is solvents). The viscosity performance versustemperature is set forth in FIG. 1.

An additional fluid of the present invention and three comparativefluids were formulated and their viscosity performance versustemperature tested. A fluid of the present invention was formulated asfollows: 1.1 wt % EAB/0.14 wt % DAXAD 19/0.15 wt % OAED in 2 wt % KCl(balance is solvents). Three comparative fluids were formulated asfollows: I) 1.1 wt % EAB/0.15 wt % OAED in 2 wt % KCl (balance issolvents); II) 1.1 wt % EAB/0.14 wt % DAXAD 19 in 2 wt % KCl (balance issolvents); and III) 1.1 wt % EAB/0.14 wt % of DAXAD 19/0.15 wt %Alkaquat DMB-451 (benzyl triammonium chloride made by Rhodia Inc.) in 2wt % KCl (balance is solvents). The viscosity performance versustemperature is set forth in FIG. 2.

It should be understood that the foregoing description is onlyillustrative of the present invention. Various alternatives andmodifications can be devised by those skilled in the art withoutdeparting from the invention. Accordingly, the present invention isintended to embrace all such alternatives, modifications and variancesthat fall within the scope of the appended claims.

1. A viscoelastic fluid, comprising: one or more cationic surfactants;one or more anionic polymers; one or more zwitterionic and/or amphotericsurfactants; and water; wherein the one or more cationic surfactantsincludes methylpolyoxyethylene octadecanammonium chloride wherein thezwitterionic surfactant has the following structural formula:

wherein R₁, R₂, and R₃ are as described above; R₄ is a hydrocarbylradical with a chain length 1 to 4; and wherein the amphotericsurfactant has the following structural formula:

wherein R₁, R₂, and R₄ are as described above.
 2. The fluid of claim 1,further comprising a component selected from the group consisting oforganic acids, organic acid salts, inorganic salts, and combinationsthereof.
 3. The fluid of claim 2, wherein the component is selected fromthe group consisting of potassium chloride and trimethylammoniumchloride.
 4. The fluid of claim 2, wherein the component is present atabout 0.1 wt % to about 10 wt %.
 5. The fluid of claim 2, furthercomprising a proppant.
 6. The fluid of claim 1, wherein the one or moreanionic polymers has the following structural formula:

wherein n is an integer from about 8 to about
 100. 7. The fluid of claim6, wherein the one or more anionic polymers is present at from about 0.1wt % to about 5 wt %.
 8. The fluid of claim 1, wherein the one or moreanionic polymers has the following structural formula:

wherein n is an integer from about 8 to about
 100. 9. The fluid of claim8, wherein the one or more anionic polymers is present at from about 0.1wt % to about 5 wt %.
 10. The fluid of claim 1, wherein the one or moreanionic polymers is present at from about 0.1 wt % to about 5 wt %. 11.The fluid of claim 10, wherein R₁ is an alkyl or alkenyl group havingabout 18 to about 22 carbon atoms, and wherein R₂, R₃ and R₅,independently, each have about 1 to about 6 carbon atoms.
 12. The fluidof claim 1, wherein the one or more cationic surfactants is present atfrom about 1 to about 40 wt % based on weight of active surfactant. 13.The fluid of claim 1, wherein the one or more cationic surfactants ispresent at from about 5 to about 20 wt % based on weight of activesurfactant.
 14. The fluid of claim 1, wherein the one or morezwitterionic surfactants is selected from the group consisting of thefollowing structural formulas:

and combinations thereof.
 15. The fluid of claim 1, wherein the one ormore zwitterionic surfactants is erucyl amidopropyl betaine.
 16. Thefluid of claim 1, wherein the water is present at about 50 wt % or morebased on fluid weight.
 17. The fluid of claim 1, wherein the water ispresent at about 70 wt % or more based on fluid weight.
 18. The fluid ofclaim 1, wherein the one or more cationic surfactants and the one ormore zwitterionic and/or amphoteric surfactants are present at about 0.1to about 10 wt % based on fluid weight.
 19. The fluid of claim 1,wherein the one or more cationic surfactants and the one or morezwitterionic and or amphoteric surfactants are present at about 0.25 toabout 2 wt % based on fluid weight.
 20. The fluid of claim 1, whereinthe one or more cationic surfactants and the one or more zwitterionicand/or amphoteric surfactants are present at less than 2 wt % based onfluid weight.
 21. The fluid of claim 1, further comprising a proppant.22. The fluid of claim 1, wherein the one or more cationic surfactantsis present at from about 1 to about 40 wt % based on weight of activesurfactant, wherein the anionic polymer is present at from about 0.1 wt% to about 5 wt % based on fluid weight, wherein the water is present atabout 50 wt % or more based on fluid weight, wherein the one or morecationic surfactants and the one or more zwitterionic and/or amphotericsurfactants are present at about 0.1 to about 10 wt % based on fluidweight, the fluid further comprising a component selected from the groupconsisting of organic acids, organic acid salts, inorganic salts, andcombinations thereof, wherein the component is present at about 0.1 wt %to about 10 wt % based on fluid weight.
 23. The fluid of claim 1,wherein R₁ is an alkyl or alkenyl group having about 18 to about 22carbon atoms, and wherein R₂, R₃ and R₅, independently, each have about1 to about 6 carbon atoms.